Field of Technology
The present disclosure relates to an austenitic stainless steel. In particular, the present disclosure relates to a cost-effective stabilized austenitic stainless steel composition including, inter alia, a low Ni level, no more than an incidental level of Mo, high yield strength, and at least comparable corrosion resistance and high temperature properties relative to certain higher Ni austenitic alloys such as, for example, T-321 alloy (UNS S32100).
Description of the Background of the Technology
Austenitic stainless steels exhibit a combination of highly desirable properties that make them useful for a wide variety of industrial applications. These steels possess a base composition of Fe that is balanced by the addition of austenite-promoting and stabilizing elements, such as Ni, Mn, and N, to allow additions of ferrite-promoting elements, such as Cr and Mo, which enhance corrosion resistance, to be made while maintaining an austenitic structure at room temperature. The austenitic structure provides the steel with highly desirable mechanical properties, particularly toughness, ductility, and formability.
A specific example of an austenitic stainless steel is AISI Type 316 stainless steel (UNS S31600), which is a 16-18% Cr, 10-14% Ni, and 2-3% Mo-containing alloy. The ranges of alloying ingredients in this alloy are maintained within the ranges specified in order to maintain a stable austenitic structure. As is understood by one skilled in the art, Ni, Mn, Cu, and N contents, for example, contribute to the stability of the austenitic structure. However, the rising costs of Ni and Mo have created the need for cost-effective alternatives to S31600 which still exhibit high corrosion resistance and good formability.
Another alloy alternative is Grade 216 (UNS S21600), which is described in U.S. Pat. No. 3,171,738. S21600 contains 17.5-22% Cr, 5-7% Ni, 7.5-9% Mn, and 2-3% Mo. Although S21600 is a lower Ni, higher Mn variant of S31600, the strength and corrosion resistance properties of S21600 are much higher than those of S31600. However, as with the duplex alloys, the formability of S21600 is not as good as that of S31600. Also, because S21600 contains the same amount of Mo as does S31600, there is no cost savings for Mo.
A variant of S31600 also exists which is primarily intended for use at high temperatures. This alloy is designated as Type 316Ti (UNS S31635). The significant difference between S31600 and S31635 is the presence of a small addition of titanium balanced to the amount of C and N present in the steel. The resulting steel, S31635, is less prone to the deleterious formation of Cr carbides at elevated temperatures and during welding, a phenomenon known as sensitization. Such additions can also enhance elevated temperature properties due to the strengthening effects of primary and secondary carbide formation. The specified range for titanium in S31635 is given by the following equation:[5×(% C+% N)]≦Ti≦0.70%.However, S31635 uses costly raw material.
Another example of an austenitic stainless steel is Type 321 stainless steel (UNS S32100), which includes, in weight percentages, 17.00-19.00% Cr, 9.00-12.00% Ni, up to 2.00% Mn, up to 0.08% C, up to 0.75% Si, [5×(% C+% N)]≦T≦0.70%, up to 0.045% P, up to 0.030% S, up to 0.10% N, and balance Fe. Type 321 alloy is stabilized against Cr carbide formation by the addition of titanium balanced to the amount of C and N present in the steel. Although Type 321 alloy does not include an intentional addition of Mo, it does include a significant level of costly alloying elements.
Other examples of alloys include numerous stainless steels in which Ni is replaced with Mn to maintain an austenitic structure, such as is practiced with Type 201 steel (UNS S20100) and similar grades. However, a need remains for a corrosion resistant, lower cost alternative to relatively highly alloyed austenitic stainless steels such as Type 321 alloy and that provides at least comparable strength and hardness properties to Type 321 alloy.